Many helmet typologies, for example motorcycle helmets, ski helmets, climbing helmets, horse riding helmets, aviation and nautical sports helmets, some types of cycling and also work hard hats or bulletproof helmets incorporate an element or part commonly known as shell (known as “shell” in English), this being the outer casing of the helmet. Said shell element has various main functions, one of them being that of distributing the forces applied at some points on the surface of said shell as a consequence of a hypothetical impact in a large enough area of the user's skull, such that the levels of generated mechanical stress are lower and thus prevent or minimize biomedical damage or injuries.
In conjunction with the impact absorbing element inside the shell, commonly manufactured in expanded polystyrene (EPS) foam, the shell is responsible for managing the deceleration process of the impact, which allows lengthening the deceleration stroke by means of the thickness of the helmet, such that the deceleration pulse experienced by the encephalic mass of the user is sufficiently less than that which it would experience without said helmet, below specific maximum acceleration and pulse amplitude parameters, thus reducing the consequent inertial forces and preventing or minimizing biomedical damage or injury.
Again, in conjunction with the impact absorbing element, another function of the shell is to absorb a large part of the initial kinetic energy of the impact by means of helmet deformation or destruction work, such that the final kinetic energy is reduced, thus minimizing the rebound velocity and the need of managing a higher deceleration, reducing the elastic component of the impact.
The shell is also the component of the helmet responsible for withstanding possible stress abrasion due to the helmet surface sliding on or against the impact surface, thus reducing the transmission of these stresses to the user, reducing or minimizing biomedical damage or injuries, as well as the component acting as functional and structural support of all the parts of the helmet during regular use.
It is worth mentioning that the shells described above should not be mistaken with the shells of other helmet typologies, such as most bicycle helmets used today and certified according to EN 1078, for example, the function of which is merely as a finish, given that the functions described above are satisfied to the extent required for said application and regulation by the inner impact absorption element. Said shells, manufactured from thermoplastic sheets or films having a very small thickness, less than 0.5 mm, by means of thermoforming processes or the like, are not within the scope of application of the invention.
The functions described above for the shell involve the need for said shell to provide a series of generic mechanical properties by means of its geometric structure and constituent materials, such as:                rigidity or ability to withstand the application of impact forces maintaining a sufficiently contained deformation level according to the application and regulation to be complied with, regardless of testing conditions such as temperature or type of impact anvil applied;        plasticity or ability to be permanently deformed to a point of no return when stresses above the elastic range thereof are applied to it;        toughness or ability to absorb energy before breaking; and        abrasion resistance.        
For these requirements, an ideal material for the shell would have a very vertical stress-deformation diagram and a very large and planar plastic area before breaking.
The quantification of said generic mechanical properties will depend on the product typology and on the testing specifications of the regulation to be applied and must, therefore, be modulated accordingly.
In contrast to the mechanical requirements imposed for the product, it is necessary and appropriate to limit the total weight of the shell element for the sake of ergonomics and comfort (also for promoting helmet use). The concept of using a composite material, particularly a fiber reinforced thermoplastic matrix material grows out of the dispute between the need for mechanical performance and weight.
Document WO2007045466-A1 describes using a composite material in the intermediate section of the hollow body basically constituting a fuel tank for vehicles with an internal combustion engine. Said section is formed by the superposition of multiple layers formed by portions of sheets or fabrics preimpregnated with polymeric resin of which the last layer, the outermost, is formed by at least one band of portions of sheets or portions of weave fabric, twill or plain type, in carbon fiber. Although the body constituting the tank is a body provided with a hollow and an opening, as occurs with a shell, the opening of the tank is considerably smaller than the maximum diameter of the cavity of the tank and when added to the resistance-related requirements of respective regulations, manufacturing a shell by the standard methods used for fuel tanks becomes unviable.
A particular type of materials that seems to satisfy the needs to be complied with by the shell are those known with the acronym LFRTP, i.e., Long Fiber Reinforced Thermoplastic, or CFRTP, i.e., Continuous Fiber Reinforced Thermoplastic.
One of the basic components of an LFRTP-type material is the reinforcement fabric, the material used usually being made of glass fiber, and/or aramid fiber and/or carbon fiber. With respect to the structure of the fabric, it can be a felt-type fabric, with fibers without a specific orientation joined together by means of a binder; a woven fabric, in which, depending on the arrangement of the weft and warp yarns, can be distinguished between a plain fabric, a twill fabric, a satin fabric, a unidirectional fabric (with most of the yarns aligned in one direction) and a multiaxial fabric. In addition to the structure of the fabric, the fabrics can be superposed in various manners, for example each layer following a different direction or combining certain types of fabrics with others, depending on the application.
The other basic component of any polymeric composite material is the matrix, the most usual matrix being thermosetting, those having an epoxy base, a polyester base, a vinylester base, an acrylic base, a phenolic base and a polyurethane base being distinguished, among others. In contrast to thermosetting matrices, there are also thermoplastic matrices from the group formed by polypropylenes, polyamides, polyethylene-terephthalates, polybutylene-terephthalates, polycarbonates, polyphenylene oxides, polyoxymethylenes, polyurethanes, etc., which are those classified under the name “LFRTP composite”.
The production and automation in manufacturing continuous fiber reinforced products using thermosetting resins as a matrix is relatively simple due to the low viscosity they have prior to curing because this allows for an easy impregnation of the fiber using low pressures, below 10 bars, which allows using relatively unsophisticated, lower cost processes of manufacture.
However, in comparison with thermoplastic resins, thermosetting matrices have serious limitations, such as low productivity because they require a long time for complete curing, involve processes entailing certain fouling because the resin adheres throughout all the equipment and installations, and particularly during processing, very large amounts of volatile organic compound emissions which are very hazardous to the health of operators are produced which entail increasing process difficulties for the prevention thereof as occupational safety regulations establish more restrictive limits. Additionally, there is a limited number of resins having a very limited toughness and which furthermore are not recyclable. On the other hand, the impregnation of continuous fibers using thermoplastic materials is very complicated due to their high viscosity and low ability of the fibers to become wet.
Using LFRTP fibers in manufacturing a rigid body made of composite material that has at least one continuous surface that is smooth, such as a shell, is known through document US2010/0209683-A1. The process involves applying a series of fabrics of thermoplastic fibers forming a mat to a substrate made of a mixture of thermo-fusible fibers and other fibers that do not react at the same melting temperature as the thermo-fusible fibers of the mixture for the purpose of forming a multilayer structure, and subjecting the multilayer structure that is so formed to a cycle of heating and compression while at the same time bringing the set of fabrics of thermoplastic fibers of the structure in contact with a continuous and smooth heating surface that is part of a heating system in order to form the rigid portion of composite material. The proposed substrate comprises thermosetting components selected from the group consisting of polyvinyl esters, phenolic resins, unsaturated polyesters and epoxy. The thermoplastic fibers of the mat are selected from the group of thermoplastic fibers consisting of polypropylenes, polyesters and co-polyesters, polyamides, polyethylene, polyvinyl chloride and polyphenylene sulfide. The other fibers of the substrate comprise thermoplastic fiber.
According to the examples described in said document, the process basically consists of thermoforming, consisting of arranging portions of fiber reinforced LFRTP fabrics on a mold reproducing the geometry of the shell of a helmet forming several layers, one on top of the other, heating the multilayer structure and applying pressure under vacuum for a certain time. Cooling to a temperature which allows demolding is then performed. The thermoforming process used in this document does not allow manufacturing shells having an almost closed geometry, so applying said process is ineffective in helmet typology the shells of which must comply with the main functions described above.
It would therefore be desirable to have a method of manufacture that allows effectively, rapidly and economically producing bodies having an almost closed geometry such as the shells described above.